1. Field of the Invention
The present invention relates to a semiconductor laser device and a method of fabricating the same, and more particularly, it relates to a semiconductor laser device formed by mounting a semiconductor laser element on a base in a junction-down system and a method of fabricating the same.
2. Description of the Background Art
A semiconductor laser device formed by mounting a semiconductor laser element on a submount (base) in a junction-down system is known in general. The junction-down system is a method of fixing a surface of the semiconductor laser element closer to an emission layer to the submount.
FIG. 17 is a sectional view showing a semiconductor laser element 100 having a plurality of ridge portions according to first prior art taken along a direction perpendicular to a cavity. The structure of the semiconductor laser element 100 having a plurality of ridge portions according to the first prior art is described with reference to FIG. 17.
In the semiconductor laser element 100 having a plurality of ridge portions according to the first prior art, an n-type buffer layer 102 of n-type GaInP having a thickness of about 0.3 xcexcm, an n-type cladding layer 103 of n-type AlGaInP having a thickness of about 2 xcexcm, a multiple quantum well (MQW) emission layer 104 of GaInP/AlGaInP and a p-type first cladding layer 105 of p-type AlGaInP having a thickness of about 0.3 xcexcm are successively formed on an n-type GaAs substrate 101, as shown in FIG. 17.
A mesa (trapezoidal) ridge portion constituted by a p-type second cladding layer 106 of p-type AlGaInP having a thickness of about 1.2 xcexcm and a p-type contact layer 107 of p-type GaInP having a thickness of about 0.1 xcexcm is formed on the central portion of the p-type first cladding layer 105. This ridge portion is in the form of a stripe having a bottom portion of about 2.5 xcexcm in width. Dummy ridge portions similar in structure to the ridge portion are formed to hold the ridge portion located at the center therebetween at prescribed intervals.
An n-type optical confinement layer 108 of n-type AlInP having a thickness of about 0.3 xcexcm and an n-type current blocking layer 109 of n-type GaAs having a thickness of about 0.5 xcexcm are formed to cover the upper surface of the p-type first cladding layer 105 and the upper and side surfaces of the dummy ridge portions located on the right and left sides while exposing only the upper surface of the central ridge portion. Therefore, no current flows to the dummy ridge portions. A p-type cap layer 110 of p-type GaAs having a thickness of about 3 xcexcm is formed to cover the upper surface of the central ridge portion and the overall upper surface of the n-type current blocking layer 109.
A p-side electrode 111 consisting of a multilayer film of a Cr layer having a thickness of about 0.1 xcexcm and an Au layer having a thickness of about 3 xcexcm is formed on the p-type cap layer 110. The p-side electrode 111 is formed to have a shape comprising recess portions and projection portions reflecting the shapes of the ridge portion and the dummy ridge portions, while parts of the p-side electrode 111 located on the dummy ridge portions are formed on positions higher than a part of the p-side electrode 111 located on the upper surface of the central ridge portion by the thicknesses of the n-type optical confinement layer 108 and the n-type current blocking layer 109. An n-side electrode 112 consisting of a multilayer film of an Auxe2x80x94Ge layer having a thickness of about 0.2 xcexcm, an Ni layer having a thickness of about 0.01 xcexcm and an Au layer having a thickness of about 0.5 xcexcm is formed on the back surface of the n-type GaAs substrate 101.
FIG. 18 is a sectional view showing the semiconductor laser element 100 according to the first prior art shown in FIG. 17 in a state mounted on a submount 113 in a junction-down system. Referring to FIG. 18, the semiconductor laser element 100 according to the first prior art is mounted on the submount (base) 113 set on a stem (not shown) while directing the p-side electrode 111 formed on the surface thereof downward in the junction-down system. A metal film 114 consisting of Ti, Pt and Au is formed on an aluminum nitride layer provided on the upper surface of the submount 113. A low melting point metal layer 115 of Pbxe2x80x94Sn 60% or Agxe2x80x94Sn 95% serving as a fusing material is formed on the metal film 114.
In order to mount the semiconductor laser element 100 on the submount 113 while directing the p-side electrode 111 downward in the junction-down system, the low melting point metal layer 115 serving as the fusing material bonds (welds) projection portions of the p-side electrode 111 to the submount 113. In this case, voids 116 are formed between recess portions of the p-side electrode 111 and the low melting point metal layer 115.
FIG. 19 is a sectional view of a semiconductor laser element 120 having a single ridge portion according to second prior art taken along a direction perpendicular to a cavity. The structure of the semiconductor laser element 120 having a single ridge portion according to the second prior art is now described with reference to FIG. 19.
In the semiconductor laser element 120 having a single ridge portion according to the second prior art, an n-type buffer layer 102, an n-type cladding layer 103, an MQW emission layer 104 and a p-type first cladding layer 105 are successively formed on an n-type GaAs substrate 101, similarly to the semiconductor laser element 100 according to the first prior art shown in FIG. 17. The thicknesses and compositions of these layers 102 to 105 are similar to those of the semiconductor laser element 100 according to the first prior art shown in FIG. 17.
A mesa (trapezoidal) ridge portion consisting of a p-type second cladding layer 121 of p-type AlGaInP having a thickness of about 1.2 xcexcm and a p-type contact layer 122 of p-type GaInP having a thickness of about 0.1 xcexcm is formed on the p-type first cladding layer 105. This ridge portion is in the form of a stripe having a bottom portion of about 2.5 xcexcm in width.
An n-type optical confinement layer 123 of n-type AlInP having a thickness of about 0.3 xcexcm and an n-type current blocking layer 124 of n-type GaAs having a thickness of about 0.5 xcexcm are formed to cover the upper surface of the p-type first cladding layer 105 while exposing only the upper surface of the ridge portion. A p-type cap layer 125 of p-type GaAs having a thickness of about 3 xcexcm is formed to cover the upper surface of the ridge portion and the overall upper surface of the n-type current blocking layer 124.
A p-side electrode 126 consisting of a multilayer film of a Cr layer having a thickness of about 0.1 xcexcm and an Au layer having a thickness of about 3 xcexcm is formed on the p-type cap layer 125. The p-side electrode 126 is formed to have a shape comprising recess portions and projection portions reflecting the shape of the ridge portion. An n-side electrode 127 consisting of a multilayer film of an Auxe2x80x94Ge layer having a thickness of about 0.2 xcexcm, an Ni layer having a thickness of about 0.01 xcexcm and an Au layer having a thickness of about 0.5 xcexcm is formed on the back surface of the n-type GaAs substrate 101.
FIG. 20 is a sectional view showing the semiconductor laser element 120 according to the second prior art shown in FIG. 19 in a state mounted on a submount 113 in the junction-down system. Referring to FIG. 20, the semiconductor laser element 120 according to the second prior art is mounted on the submount (base) 113 set on a stem (not shown) while directing the p-side electrode 126 formed on the surface thereof downward in the junction-down system. A metal film 114 consisting of Ti, Pt and Au is formed on an aluminum nitride layer provided on the upper surface of the submount 113. A low melting point metal layer 115 of Pbxe2x80x94Sn 60% or Agxe2x80x94Sn 95% for serving as a fusing material is formed on the metal film 114.
In order to mount the semiconductor laser element 120 on the submount 113 while directing the p-side electrode 126 downward in the junction-down system, the low melting point metal layer 115 serving as the fusing material bonds (welds) projection portions of the p-side electrode 126 to the submount 113. In this case, voids 117 are formed between recess portions of the p-side electrode 126 and the low melting point metal layer 115.
FIG. 21 is a sectional view of a semiconductor laser element 130 having a non-current injection region on a cavity end surface according to third prior art taken along a direction perpendicular to the cavity. FIG. 22 is an enlarged sectional view showing a portion around the non-current injection region of the semiconductor laser element 130 according to the third prior art shown in FIG. 21 in a direction parallel to the cavity. The structure of the semiconductor laser element 130 having the non-current injection region on the cavity end surface according to the third prior art is described with reference to FIGS. 21 and 22.
In the semiconductor laser element 130 having the non-current injection region on the cavity end surface according to the third prior art, an n-type buffer layer 102, an n-type cladding layer 103, an MQW emission layer 104 and a p-type first cladding layer 105 are successively formed on an n-type GaAs substrate 101, similarly to the semiconductor laser element 100 according to the first prior art shown in FIG. 17. The thicknesses and compositions of these layers 102 to 105 are similar to those of the semiconductor laser element 100 according to the first prior art shown in FIG. 17.
A mesa (trapezoidal) ridge portion consisting of a p-type second cladding layer 121 of p-type AlGaInP having a thickness of about 1.2 xcexcm and a p-type contact layer 122 of p-type GaInP having a thickness of about 0.1 xcexcm is formed on the p-type first cladding layer 105. This ridge portion is in the form of a stripe having a bottom portion of about 2.5 xcexcm in width.
An n-type optical confinement layer 131 of n-type AlInP having a thickness of about 0.3 xcexcm is formed to cover the upper surface of the p-type first cladding layer 105 while exposing only the upper surface of the ridge portion. An n-type current blocking layer 132 of n-type GaAs having a thickness of about 0.5 xcexcm is formed to cover substantially the overall upper surface of the n-type optical confinement layer 131 and a region (see FIG. 22) of the exposed upper surface of the ridge portion close to the cavity end surface. The non-current injection region is formed under the part of the n-type current blocking layer 132 formed on the region (see FIG. 22) of the upper surface of the ridge portion close to the cavity end surface. A p-type cap layer 133 of p-type GaAs having a thickness of about 3 xcexcm is formed to cover the upper surfaces of the ridge portion and the n-type current blocking layer 132.
A p-side first electrode 134 consisting of a multilayer film of a Cr layer having a thickness of about 0.1 xcexcm and an Au layer having a thickness of about 1 xcexcm is formed on the p-type cap layer 133. A p-side second electrode 135 consisting of a multilayer film of a Pd layer having a thickness of about 0.1 xcexcm and an Au layer having a thickness of about 2 xcexcm is formed on a region of the p-side first electrode 134 other than the non-current injection region. The p-side second electrode 135 has a shape comprising recess portions and projection portions reflecting the shape of the ridge portion. An n-side electrode 136 consisting of a multilayer film of an Auxe2x80x94Ge layer having a thickness of about 0.2 xcexcm, an Ni layer having a thickness of about 0.01 xcexcm and an Au layer having a thickness of about 0.5 xcexcm is formed on the back surface of the n-type GaAs substrate 101.
FIG. 23 is a sectional view showing the semiconductor laser element 130 according to the third prior art shown in FIG. 21 in a state mounted on a submount 113 in the junction-down system. Referring to FIG. 23, the semiconductor laser element 130 according to the third prior art is mounted on the submount (base) 113 set on a stem (not shown) while directing the p-side second electrode 135 formed on the surface thereof downward in the junction-down system. A metal film 114 consisting of Ti, Pt and Au is formed on an aluminum nitride layer provided on the upper surface of the submount 113. A low melting point metal layer 115 of Pbxe2x80x94Sn 60% or Agxe2x80x94Sn 95% for serving as a fusing material is formed on the metal film 114.
In order to mount the semiconductor laser element 130 on the submount 113 while directing the p-side second electrode 135 downward in the junction-down system, the low melting point metal layer 115 serving as the fusing material bonds (welds) projection portions of the p-side second electrode 135 to the submount 113. In this case, voids 118 are formed between regions of the p-side second electrode 135 other than projection portions and the low melting point metal layer 115.
In a semiconductor laser device fabricated by mounting the semiconductor laser element 100, 120 or 130 according to the first, second or third prior art on the submount 113 in the junction-down system, as hereinabove described, the low melting point metal layer 115 provided on the submount 113 for serving as the fusing material bonds the semiconductor laser element 100, 120 or 130 to the submount 113. In general, the submount 113 also serves as a heat sink absorbing heat of the semiconductor laser element 100, 120 or 130 and radiating the same outward.
In the semiconductor laser device fabricated by bonding the semiconductor laser element 100 according to the aforementioned first prior art to the submount 113 in the junction-down system, however, heat radiation as well as bond strength are disadvantageously reduced due to the voids 116 formed between the recess portions of the p-side electrode 111 and the low melting point metal layer 115, as shown in FIG. 18. Therefore, the semiconductor laser device having the semiconductor laser element 100 according to the first prior art is disadvantageously reduced in reliability.
In the semiconductor laser device fabricated by bonding the semiconductor laser element 120 or 130 according to the aforementioned second or third prior art to the submount 113 in the junction-down system, only the projection portions of the p-side electrode 126 or the p-side second electrode 135 come into contact with the metal film 114 formed on the submount 113 as shown in FIG. 20 or 23, and hence stress is disadvantageously applied to the ridge portion located under the projection portions of the p-side electrode 126 or the p-side second electrode 135. Consequently, operating current and operating voltage may be increased due to the stress. Further, the voids 117 or 118 are formed between the regions of the p-side electrode 126 or the p-side second electrode 135 other than the projection portions and the low melting point metal layer 115, and hence heat radiation as well as bond strength are disadvantageously reduced. When the bond strength is reduced, the semiconductor laser element 120 or 130 tends to incline when mounted on the submount 113. Thus, the semiconductor laser device having the semiconductor laser element 120 or 130 according to the second or third prior art is disadvantageously reduced in reliability.
An object of the present invention is to provide a semiconductor laser device capable of improving reliability in a structure obtained by mounting a semiconductor laser element on a submount (base) in a junction-down system.
Another object of the present invention is to improve heat radiation and bond strength in the aforementioned semiconductor laser device.
Still another object of the present invention is to prevent operating current and operating voltage from increase resulting from stress applied to a ridge portion in the aforementioned semiconductor laser device.
A further object of the present invention is to provide a method of fabricating a semiconductor laser device capable of improving reliability in a structure obtained by mounting a semiconductor laser element on a submount (base) in a junction-down system.
In order to attain the aforementioned objects, a semiconductor laser device according to a first aspect of the present invention comprises a first electrode layer formed on the surface of a semiconductor element including an emission layer to have a shape comprising recess portions and projection portions, a base mounted with the semiconductor element, and a plurality of low melting point metal layers provided between the first electrode layer formed on the surface of the semiconductor element and the base for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other.
The semiconductor laser device according to the first aspect is provided with the plurality of low melting point metal layers for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other as hereinabove described, whereby the plurality of low melting point metal layers easily embed clearances resulting from the shape, comprising recess portions and projection portions, of the surface of the semiconductor element located on the bonded surfaces of the first electrode layer formed on the surface of the semiconductor element and the base, dissimilarly to a case of employing a single low melting point metal layer. Therefore, excellent heat radiation can be attained while bond strength can be improved. Thus, the bond strength can be so improved that the semiconductor element can be stably mounted on the base with no inclination. When the low melting point metal layers are prepared from a soft material, the plurality of low melting point metal layers prepared from the soft material can embed a ridge portion provided on a semiconductor laser element, thereby effectively relaxing stress applied to the ridge portion. Consequently, operating current and operating voltage can be prevented from increase resulting from stress, whereby the semiconductor laser device can attain excellent reliability.
In the aforementioned semiconductor laser device according to the first aspect, the plurality of low melting point metal layers are preferably formed to bond the first electrode layer formed on the surface of the semiconductor element and the base to each other while embedding the shape comprising recess portions and projection portions. According to this structure, excellent heat radiation can be easily attained and bond strength can be improved.
In the aforementioned semiconductor laser device according to the first aspect, the plurality of low melting point metal layers preferably have a thickness exceeding the height of the projection portions of the shape comprising recess portions and projection portions, i.e., a thickness exceeding the difference between the height of the bottom surfaces of the recess portions and the top surfaces of the projection portions. According to this structure, the plurality of low melting point metal layers can easily embed the shape comprising recess portions and projection portions.
A semiconductor laser device according to a second aspect of the present invention comprises a first electrode layer formed on the surface of a semiconductor element including an emission layer to have a shape comprising recess portions and projection portions, a base mounted with the semiconductor element, and a low melting point metal layer provided between the first electrode layer formed on the surface of the semiconductor element and the base and formed on a portion for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other to embed the shape comprising recess portions and projection portions.
The semiconductor laser device according to the second aspect is provided with the low melting point metal layer on the portion bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other to embed the shape comprising recess portions and projection portions as hereinabove described, whereby excellent heat radiation can be attained while bond strength can be improved. Thus, the bond strength can be so improved that the semiconductor element can be stably mounted on the base with no inclination. When the low melting point metal layer is prepared from a soft material, the low melting point metal layer prepared from the soft material can embed a ridge portion provided on a semiconductor laser element, thereby effectively relaxing stress applied to the ridge portion. Consequently, operating current and operating voltage can be prevented from increase resulting from stress, whereby the semiconductor laser device can attain excellent reliability.
In the aforementioned semiconductor laser device according to each of the first and second aspects, the low melting point metal layer(s) preferably include(s) a first low melting point metal layer provided on the first electrode layer formed on the semiconductor element, and a second low melting point metal layer provided on the base. According to this structure, the first low melting point metal layer is so melted as to embed the shape, comprising recess portions and projection portions, of the first electrode layer formed on the surface of the semiconductor element, so that excellent heat radiation can be attained and bond strength can be improved. In this case, the first low melting point metal layer may include at least either an Sn layer or an Auxe2x80x94Sn layer, and the second low melting point metal layer may include at least any of a Pbxe2x80x94Sn layer, an Agxe2x80x94Sn layer and an Auxe2x80x94Sn layer. Further, the first low melting point metal layer may include a multilayer film having a plurality of Auxe2x80x94Sn layers of different Auxe2x80x94Sn compositions.
In the aforementioned semiconductor laser device according to each of the first and second aspects, the first electrode layer preferably includes a first electrode layer provided on the surface of the semiconductor element, a second electrode layer is preferably further provided on the surface of the semiconductor element, and the low melting point metal layer(s) preferably bond(s) the first electrode layer and the second electrode layer to the base. According to this structure, bond strength between the second electrode layer and the base can also be easily improved.
In the aforementioned semiconductor laser device according to each of the first and second embodiments, the semiconductor element including the emission layer is preferably formed on a first conductivity type GaN substrate. According to this structure, the semiconductor element including the emission layer having excellent crystallinity can be formed on the GaN substrate.
In the aforementioned semiconductor laser device according to each of the first and second aspects, the surface of the semiconductor element closer to the emission layer is preferably mounted on the base. According to this structure of junction-down assembly, heat generated from the emission layer can be excellently radiated toward the base.
A semiconductor laser device according to a third aspect of the present invention comprises a first electrode layer formed on the surface of a semiconductor element including an emission layer to have a shape comprising recess portions and projection portions, a base mounted with the semiconductor element, and a plurality of low melting point metal layers provided between the first electrode layer formed on the surface of the semiconductor element and the base for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other, and the low melting point metal layers include a first low melting point metal layer provided on the first electrode layer formed on the surface of the semiconductor element and a second low melting point metal layer provided on the base.
The semiconductor laser device according to the third aspect is provided with the first low melting point metal layer and the second low melting point metal layer for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other as hereinabove described, whereby the first and second low melting point metal layers can easily embed clearances resulting from the shape comprising recess portions and projection portions on the surface of the semiconductor element located on the bonded surfaces of the first electrode layer formed on the surface of the semiconductor element and the base, dissimilarly to a case of employing a single low melting point metal layer. Therefore, excellent heat radiation can be attained and bond strength can be improved. Thus, the bond strength can be so improved that the semiconductor element can be stably mounted on the base with no inclination. When the low melting point metal layers are prepared from a soft material, the low melting point metal layers prepared from the soft material can embed a ridge portion provided on a semiconductor laser element, thereby effectively relaxing stress applied to the ridge portion. Consequently, operating current and operating voltage can be prevented from increase resulting from stress, whereby the semiconductor laser device can attain excellent reliability.
A method of fabricating a semiconductor laser device according to a fourth aspect of the present invention comprises steps of forming a first low melting point metal layer on a first electrode layer formed on the surface of a semiconductor element including an emission layer to have a shape comprising recess portions and projection portions, forming a second low melting point metal layer on a base mounted with the semiconductor element, and heating the first low melting point metal layer and the second low melting point metal layer in an opposed state thereby melting the first low melting point metal layer and the second low melting point metal layer and bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other.
In the method of fabricating a semiconductor laser device according to the fourth aspect, the first low melting point metal layer formed on the first electrode layer and the second low melting point metal layer formed on the base are melted for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other as hereinabove described, whereby the first and second low melting point metal layers easily embed clearances resulting from the shape comprising recess portions and projection portions on the surface of the semiconductor element located on the bonded surfaces of the first electrode layer formed on the surface of the semiconductor element and the base, dissimilarly to a case of employing a single low melting point metal layer. Therefore, excellent heat radiation can be attained and bond strength can be improved. Thus, the bond strength can be so improved that the semiconductor element can be stably mounted on the base with no inclination. When the low melting point metal layers are prepared from a soft material, the low melting point metal layers prepared from the soft material can embed a ridge portion provided on a semiconductor laser element, thereby effectively relaxing stress applied to the ridge portion. Consequently, operating current and operating voltage can be prevented from increase resulting from stress, whereby a semiconductor laser device having excellent reliability can be formed.
In the aforementioned method of fabricating a semiconductor laser device according to the fourth aspect, the step of bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other preferably includes a step of melting the plurality of low melting point metal layers for embedding the shape comprising recess portions and projection portions thereby bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other. According to this structure, excellent heat radiation can be easily attained and bond strength can be improved.
In the aforementioned method of fabricating a semiconductor laser device according to the fourth aspect, the step of bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other preferably includes a step of bonding the surface of the semiconductor element closer to the emission layer to the base. According to this structure of junction-down assembly, heat generated from the emission layer can be excellently radiated toward the base.
In the aforementioned method of fabricating a semiconductor laser device according to the fourth aspect, the first low melting point metal layer may include at least either an Sn layer or an Auxe2x80x94Sn layer, and the second low melting point metal layer may include at least any of a Pbxe2x80x94Sn layer, an Agxe2x80x94Sn layer and an Auxe2x80x94Sn layer. Further, the step of forming the first low melting point metal layer may include a step of forming a multilayer film including a plurality of Auxe2x80x94Sn layers having different Auxe2x80x94Sn compositions.
The aforementioned method of fabricating a semiconductor laser device according to the fourth aspect preferably further comprises a step of forming the semiconductor element including the emission layer on a first conductivity type GaN substrate in advance of the step of forming the first low melting point metal layer. According to this structure, the semiconductor element including the emission layer having excellent crystallinity can be formed on the GaN substrate.
A method of fabricating a semiconductor laser device according to a fifth aspect of the present invention comprises steps of forming a first electrode layer having a shape comprising recess portions and projection portions on a surface of a semiconductor element including an emission layer and having a shape comprising recess portions and projection portions at the surface, preparing a base mounted with the semiconductor element, and melting a low melting point metal layer to embed the shape, comprising recess portions and projection portions, of the first electrode layer while opposing the base mounted with the semiconductor element and the first electrode layer formed on the surface of the semiconductor element thereby bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other.
In the method of fabricating a semiconductor laser device according to the fifth aspect, the low melting point metal layer is melted for bonding the first electrode layer formed on the surface of the semiconductor element and the base to each other to embed the shape, comprising recess portions and projection portions, of the first electrode layer, whereby excellent heat radiation can be attained and bond strength can be improved. Thus, the bond strength can be so improved that the semiconductor element can be stably mounted on the base with no inclination. When the low melting point metal layer is prepared from a soft material, the low melting point metal layer prepared from the soft material can embed a ridge portion provided on a semiconductor laser element, thereby effectively relaxing stress applied to the ridge portion. Consequently, operating current and operating voltage can be prevented from increase resulting from stress, whereby a semiconductor laser device having excellent reliability can be formed.
The aforementioned method of fabricating a semiconductor laser device according to the fifth aspect preferably further comprises a step of forming the semiconductor element including the emission layer on a first conductivity type GaN substrate in advance of the step of forming the first electrode layer. According to this structure, the semiconductor element including the emission layer having excellent crystallinity can be formed on the GaN substrate.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.